Transmission line relay protection — II

This paper is divided into three parts, the first part consisting of a list of relay nomenclature according to function and application, the second part being a general discussion on relay protection with special reference to an investigation of the Protective Devices Committee to obtain information on transmission line protective scheme and the third part, an illustrated description of the results of the investigation. The thoroughly proved methods have been given scant consideration but the operating results of new schemes have been treated in brief detail with a statement of the condition which led to the adoption of the various schemes. Many of these schemes require the use of special apparatus which is not obtainable in the open market, though in most cases it was possible to use standard apparatus. It is quite probable that some of these special schemes will be adopted by the leading manufacturers with more or less modification. In the majority of the cases cited, the actual operating results showing the number of correct and incorrect relay operations have been given and it is probable that this is the first time that such a disclosure has been so generously and frankly made. Standard names for relays as to function and application as well as qualifying terms which always have the same meaning are very desirable. The number of types of relays now in common use are so great that considerable confusion has resulted from a previous lack of uniformity of identifying designations. This confusion will be eliminated if the manufacturers and users of relays employ the terms given in the paper, which have been approved by the Standards Committee of the Institute. The first paper of a series contemplated by the Protective Devices Committee was presented three years ago. This is the second paper. Both of these deal with transmission line relay protection, and the Committee now plans to broaden its work to include also relays for protection of apparatus with the expectation of presenting additional data before the Institute as the art progresses. The entire basis of the present paper is the experience of operating companies as reported by their engineers. The general theory of relays has been almost completely disregarded. The use of combination over-current and directional schemes has become common due to the very satisfactory results that have been obtained with these relays. Differential current schemes for parallel lines are increasing in popularity on account of their freedom from a-c. potential connections. Their use is limited, however; the differential power method using directional relays is suitable in those cases where the simpler current balance is not. The tendency appears to be in favor of balancing parallel feeders wherever possible to secure freedom from faulty operation on through faults together with quick action in case of trouble on one of the group. A number of different schemes, based generally on the same principle, is described in considerable detail. The split conductor protection, though apparently successful, does not seem to be extending greatly beyond the early installations in this country, principally on account of the high cost of the cable. Pilot wire protection, while abandoned by some operators, is being tried out in newer forms by others with some apparent likelihood of future extension. The use of ground relays has been considerably extended particularly on systems having neutrals grounded through a comparatively high resistance. The application of these ground relays with regard to current and time settings is based on the same principles that apply for the relays connected in the phase. Both over-current and directional relays have been used with apparently successful results. By energizing only in case of accidental ground these relays may be set for much lower current values than the phase relays and in some cases at lower time values. The ground relays may be connected to sheath transformers or in the residual circuit of three current transformers of the usual type. Several applications of ground relays with pilot wire connections have been reported. A very sensitive potential ground relay scheme for the protection of comparatively isolated circuits has been operating with good results for a number of years. In one case, in order to avoid the necessity of installing grounding transformers, provision has been made to ground a second phase of the bus when it is shown by potential ground relays that an accidental ground has occurred on one phase. In this way a phase short circuit is produced permitting the faulty section to be isolated by standard over-current and directional relays. An application of the combination of under-voltage and over-current relays is described. It takes into consideration the feature that the potential would be reduced proportionately as the faults is approached which adds a further degree of selectivity to the ordinary current method. Accurate calculation of short circuit currents has proved highly desirable and considerable data concerning mathematical and mechanical methods of making these calculations are described. General principles of relay application and practise concerning relay settings are discussed. Some notes are included relative to foreign practise and some of the outstanding features of installations in use in other countries have been described.